Our goal is to develop an ultra short curative regimen for hepatitis C virus (HCV) infection in OUD patients. HCV is a major co-morbidity of many OUD patients. Treating their HCV reduces their subsequent IV drug use. While currently available regimens can cure HCV in most patients in 8-12 weeks, longer in patients with advanced liver disease or drug resistance mutations that are more frequent in OUD patients, when these same regimens are used for just 4 weeks, most patients fail and they develop drug resistance mutations. Ultra short treatment regimens with high cure rates are not currently achievable, nor can they be anticipated with any of the drugs on the market or in development. We now seek to leverage our exciting ?SBIR Phase 1 equivalent? research results into a novel double nucleoside-based treatment for HCV in OUD patients. Our overall hypothesis is that Riboscience?s unique and highly synergistic double nucleoside (nuc) regimen (RBS1154, a uridine monophosphate prodrug that is more potent than Gilead?s sofosbuvir + RBS1502, an equally superior cytidine mono-phosphate prodrug) will cure patients with just one prescription in 4 weeks or less (most probably in 1-2 weeks, but those who don?t get cured can simply be treated a bit longer?as their is no risk of resistance with an all nucleoside regimen). Such a double nuc combo will have a major impact with respect to the supportive care of a large segment of the OUD population--accelerating time to cure of a frequent major comorbidity, reducing the risk of HCV spread to other OUD patients, and decreasing IV drug use. To test our hypothesis, and enable clinical studies of our double nucleoside combo in OUD patients, we seek to accomplish the following specific aims: 1) complete critical chemistry and manufacturing (CMC) and virology activities by: a) finishing RBS1502?s process chemistry and synthesize 1 Kg non-GMP of each nuc for non- GLP tox studies; b) complete primary pharmacology package (virology); 2) enable IND-enabling GLP toxicity studies by: a) synthesizing 15 Kg of GMP API of each nucleoside to support GLP tox and Phase 1 studies; b) develop analytical methods to support ADME and toxicity studies; 3) Perform in vitro and in vivo characterizations to enable GLP toxicity studies by: a) completing primary and secondary pharmacology characterizations for the U and C analogs in line with FDA guidances, including PK and in vitro and in vivo ADME studies; b) performing dose-range finding non-GLP toxicity studies for the U and C analogs; 4) complete data package to support IND filing by performing: a) 8-wk GLP toxicity studies for U and C and 4-wk U+C combination toxicity study; b) GLP safety pharmacology (respiratory, CV, CNS) and genotoxicity studies. Successful accomplishment of our aims will have a major impact on the supportive care of OUD patients. Our novel double nucleoside approach can yield the most convenient, safest, ultra short treatment option for HCV-infected OUD patients. This will be a very attractive commercial solution for OUD patients and enable securing subsequent additional private sector funding for further clinical development and commercial success.